OFDMA
Orthogonal frequency-division multiplexing (OFDM) is a modulation technique used at the physical layer (PHY) of a number of wireless networks, e.g., networks designed according to the IEEE 802.11a/g and IEEE 802.16/16e standards. Orthogonal frequency division multiple access (OFDMA) is a multiple access scheme based on OFDM. In OFDMA, separate sets of orthogonal tones (subchannels) and time slots are allocated to multiple transceivers or mobile stations (MS) so that the transceivers can communicate concurrently. OFDMA is widely adopted in many next generation cellular networks, such as 3GPP long term evolution (LTE) and, the IEEE 802.16m standard due to its effectiveness and flexibility in radio resource allocation.
OFDMA Frequency Allocation
The radio spectrum is a scarce resource in wireless communications, and therefore an efficient use is needed. The rapid growth of wireless applications and the mobile stations require a good radio resource management (RRM) scheme that can increase the network capacity, and reduce deployment cost. Consequently, there is a need to develop an effective radio frequency allocation scheme for OFDMA.
The fundamental challenge in frequency allocation is the inequality between the size of the spectrum, the large number of mobile stations, and the large geographical areas that need to be covered. In other words, the same frequency spectrum should be reused in multiple cells. This will inevitably incur inter-cell interference (ICI), when mobile stations in adjacent cells use the same spectrum. In fact, ICI has been shown to be the predominant performance-limiting factor for wireless cellular networks. As a result, a significant amount of research has been devoted to developing ICI-aware radio resource allocation for cellular networks.
Fractional Frequency Reuse
Several ICI-aware frequency allocation schemes are known for the next generation OFDMA networks. Fractional frequency reuse, (FFR) is one technique supported in WiMAX.
As shown conventionally in FIG. 1A, FFR partitions a cell 15, shown approximately as a hexagon, into a cell center 10 around a base station (BS) 11 and cell edge regions 20. The cell center of adjacent cells can use the same overlapping spectrum, while the cell edge regions of adjacent cells should use non-overlapping spectra. This is designed with the goal of striking a better trade-off between spectral efficiency and interference mitigation by leveraging the fact that cell-edge mobile stations are more ICI-prone than cell-center mobile stations. Typically, the base station 11 and other base stations communicate with each other via an infrastructure 25 or ‘backbone’ of the network.
There are many possible realizations of the FFR concept in terms of bandwidth partitioning and definition of cell center and cell edge. Most of these schemes, however, are of a fixed configuration. That is, their bandwidth allocation for cell center and cell edge is predetermined and not adaptive to dynamic traffic variations. This is not efficient when traffic changes over time and/or the number of mobile stations is asymmetric among a group of cells.
Graph-Based Framework in Prior Channel Allocation
The channel allocation problem in conventional (non-OFDMA) cellular and mesh networks has been solved using a graph coloring approach. In the conventional problem formulation, each node in the graph corresponds to a base station (BS) or an access point (AP) in the network to which subchannels are allocated. The edge connecting two nodes represents the co-channel interference, which typically corresponds to the geographical proximity of the BSs. Then, the allocation problem subject to the interference constraints becomes a node coloring problem, wherein two interfering nodes should not have the same color.
Other graph based methods are described in U.S. patent application Ser. No. 12/112,346, “Graph-Based Method for Allocating Resources in OFDMA Networks,” filed by Tao et al., on Mar. 30, 2008, and U.S. patent application Ser. No. 12/112,400, “Graph-Based Method for Allocating Resources in OFDMA Networks,” filed by Tao et al., on Mar. 30, 2008. The first Application ('346) constructs a graph and then partitions the graph into non-overlapping clusters of nodes to minimize interference. The second Application ('400) constructs a graph using diversity sets maintained at base stations, and then allocating channel resources to the mobile stations based on the structure of the graph, the potential interference between the stations, and the channel quality.